1. Field of Invention
The present invention is directed to a multilayer system for protecting components exposed to environmental and thermal conditions and, particularly, barrier coating for components for heat engines, such as gas turbine engines.
2. Background
Ceramics are commonly used in high temperature structural applications such as gas turbine engines, internal combustion engines, and heat exchangers. In particular, silicon containing ceramics are often used in such extreme conditions, but are prone to rapid recession in combustion environments in which water vapor is typically produced.
Temperature capability of structural components exposed to environmental and thermal conditions limit efficiency and emissions of gas turbine engines. Components used in gas turbine engines, which may be exposed to temperatures in excess of 1000° C., should be strong and resist corrosion due to exposure to air streams as well as abrasion and shock resistance against collision with fine particles. In particular, components such as turbine rotors, turbine blades, nozzles, combustors, scrolls, nozzle supports, seal rings, spring rings, diffusers, ducts and shrouds in the engine hot section are susceptible to coating loss from spallation or erosion.
Silicon containing ceramics typically form a slow forming silica (SiO2) scale in clean oxidizing environments. However, water vapor, molten salts, or a reducing atmosphere present in the environment may react with the silica scale to form liquid silicates. Oxygen may then diffuse through the liquid silicate and oxidize the ceramic substrate. High water vapor levels result in the formation of hydrated silica species (Si(OH)x) and subsequent evaporation of the silica scale. Oxidizing and reducing gases in complex combustion atmospheres generate SiO(g) directly or SiO2 which is reduced to SiO(g). To reduce the rate of recession, ceramics may be coated as, for example, disclosed in U.S. Pat. No. 6,759,151 issued to Lee; U.S. Pat. No. 6,682,821 issued to Fukudome et al.; and U.S. Pat. No. 6,645,649 issued to Tanaka et al.